This invention relates to a Raman amplifier.
To extend a transmission distance of a repeaterless optical transmission line and a repeater span of an optical amplifier repeater transmission line, a fiber Raman amplifier to amplify the optical signal on an optical transmission line is effective, and thereby receiving sensitivity and the SNR (the signal-to-noise ratio) are improved. For example, see T. Miyakawa et al., xe2x80x9c210 Gbit/s (10.7 Gbit/sxc3x9721 WDM) Transmission over 1200 km with 200 km Repeater Spacing for the Festoon Undersea Cable Systemxe2x80x9d, OFC 2000, Baltimore, Md., USA, Mar. 7-10, 2000 and H. Kawakami et al., xe2x80x9cHighly efficient Distributed Raman amplification system in a zero-dispersion-flattened transmission linexe2x80x9d, OAA1999 Nara, Japan, Jun. 9-11, 1999.
In fiber Raman amplifiers, there are two kinds of methods of pumping. One is a forward pumping method in which the high-output pumping light to produce Raman amplification on an optical transmission line enters onto the optical transmission line in the same direction with that of signal propagation, namely from the front, and the other is a backward pumping method in which the pumping light enters onto the optical transmission line in the opposite direction to that of the optical signal propagation, namely from the back. The Raman amplification is caused by nonlinear effects of an optical fiber. Accordingly, to increase the Raman gain, it is necessary to increase pumping power and/or employ an optical fiber with large nonlinear effects. On condition that optical fibers are of the same material and configuration, the one having the smallest core diameter has the largest nonlinear effect.
In conventional fiber Raman amplifiers, an optical fiber of a large core diameter is used for the first half part of an optical transmission line where power of the optical signal is large, and an optical fiber of a small core diameter, i.e. an optical fiber of large Raman gain coefficient, is used for the latter half part of the optical fiber where the power of the optical signal is small. Then, the pumping light is introduced into the latter optical fiber from the back in the opposite direction from that of optical signal propagation. With this operation, the weak optical signal is amplified at the latter half part of the optical transmission line, and accordingly the decrease of the optical signal level is moderated or the optical signal level is increased.
When it is possible to moderate the decrease of the optical signal level or to increase the optical signal level, to extend a repeaterless optical transmission distance or a repeater span is also possible.
However, in the Raman amplification, the accumulated optical noise power also increases in the same time. Therefore, in the conventional methods, although there is no problem in terms of the optical signal level, receiving characteristics become degraded due to the rapid deterioration of the SNR.
It is therefore an object of the present invention to provide a Raman amplifier to solve the above problems.
Another object of the present invention is to provide a Raman amplifier to offer the satisfactory SNR while keeping the necessary Raman gain.
A Raman amplifier according to this invention consists of a first optical fiber to propagate the optical signal, a second optical fiber to which the optical signal output from the first optical fiber enters, the pumping light source to generate the pumping light for Raman amplification, and an optical introducer to introduce the output light from the pumping light source into the second optical fiber from the output side of the optical signal, the Raman amplifier characterized in that the ratio of the Raman gain coefficient of the second optical fiber to that of the first optical fiber is 1/1.08 or less.
By using the above configuration, it is possible to shift the location at which the optical signal is amplified into the input side, and consequently the SNR can be improved because the accumulated optical noise amount is reduced at the input part of a receiver.
When a fiber length of a certain unit is assumed, Raman gain (dB) increases proportional to the intensity (mW or W) of the pumping light, and the Raman gain and the optical noise power caused by the Raman gain are proportional. The optical noise amount generated by the same Raman gain is unchanged. However, the optical noise at the receiver differs according to locations of the occurrence. The optical noises generated at the points near to the receiver are detected as the optical noises of almost the same level. However, when the point of the Raman gain occurrence shifts to the transmitter side, the optical noise detected at the receiver is reduced according to the shifted length toward the transmitter side. Therefore, the more the location of the Raman gain occurrence shifts to the transmitter side, the more the SNR at the receiver is improved.